Xen developer summit 2015 slides

1. Virtual Machine
Introspection with
Xen
Tamas K Lengyel
tamas@tklengyel.com

2. Virtual Machine Introspection
● Isolation
● Interpretation
● Interposition

3. Isolation
● From in-guest kernel/userspace
• Provided by Xen
• Buggy emulation blurres the line
● From trusted computing base (TCB)
• Possible via Xen Security Modules
• Move introspection system out from dom0!

4. Xen Security Modules (XSM)
● Usable since Xen 4.3
and Linux 3.8
● Disaggregate the TCB
● Available on both
x86 and ARM
● Not enabled by default

5. Interpretation
● Reconstruct
kernel/process state
● Use memory forensic
techniques
● LibVMI –
http://libvmi.com
00 00 00 00 9c 95 ba e0 7c b7 37 c1 6c 6f 6f 70
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 60 ae 27 de c0 4a 80 df
e4 95 ba e0 cc 4a 80 df c0 4a 80 df 6c b0 37 c1
40 35 8e df 03 00 00 00 07 00 00 00 5c c1 c3 e0
00 00 00 00 00 00 00 00 00 70 2a de 00 00 00 00
00 00 00 00 80 7f 33 de 50 c0 c3 e0 60 c0 c3 e0
02 00 00 00 68 c0 c3 e0 02 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 f0 c3 e0 00 00 00 00 00 a0 c3 e0
00 00 00 00 a5 26 00 00 00 00 00 00 3d 1e 00 00
00 00 00 00 02 00 00 00 90 96 ba e0 c8 8f 38 c1
d4 bf c3 e0 c8 c2 c3 e0 c8 c2 c3 e0 20 00 00 00
20 00 00 00 c8 c4 c3 e0 c8 c4 c3 e0 00 1c ba df
00 7f 33 de 00 00 00 00 58 ae 27 de 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 a0 c2 c3 e0 a0 c2 c3 e0 30 84 99 de
b8 bd c3 e0 a8 79 3f fe 00 00 00 00 00 00 00 00
struct module
state
list
name
mkobj
modinfo_attrs
version
src_version
holders_dir
syms
crcs
num_syms
...
ctors
num_ctors
module_state = MODULE_STATE_LIVE
struct list_head
next
prev
unsigned int = 0
void (*)() = NULL
char[60] = "loop"
struct module_kobject
kobj
mod
drivers_dir
mp

6. Interposition
● Trap to Xen when something of interest
happens within the guest
• Enable optional hardware traps
• CLTS, HLT, LGDT, LIDT, LLDT, LTR, SGDT, MOV from
CR3, MOV from CR8, MOV to CR0, MOV to CR3, MOV
to CR4, MOV to CR8, MOV DR, MWAIT, INT3, INT0,
MTF, etc..
• See full list in Intel SDM 3c 25.1.3

7. Interposition
● Change access
permissions in EPT
● Trap violation into
Xen
● R/W/X
● With some caveats

8. EPT caveats
“An EPT violation that occurs during as a
result of execution of a read-modify-write
operation sets bit 1 (data write). Whether it
also sets bit 0 (data read) is implementation-
specific and, for a given implementation,
may differ for different kinds of read-modify-
write operations.” - Intel SDM 3c

9. EPT caveats
● “Why can't the hardware report the true
characteristics right away?” - Jan Beulich
● “when spec says so, there is a reason but I
can't tell here. :-)” - Kevin Tian
● Well.. let's just mark all write volation as
read violation too..
● Patched in Xen 4.5

10. EPT caveats
● Requires relaxing the
EPT permissions
● Requires singlestepping
the vCPU
● Many VMEXITs not
shown in picture!
● Fixed for Xen 4.6

11. EPT caveats
● Race-condition if VM
has multiple vCPU
● No solution for this
problem prior to Xen 4.6
● New method introduced
in Xen 4.6 that solves
this: altp2m

12. altp2m
● Add support for
multiple EPTs for
second stage lookup!
● One table for
“restricted view”
● One table for “normal
view”

13. altp2m
● EPT pointer can be
swapped in the
VMCS
● No need to change
EPT PTE permissions
all the time
● No race condition

14. Interposition
● Once trapped to Xen, forward events
• Formerly known as mem_event
• Renamed and reworked as vm_event in 4.6
● Request/response via shared memory ring
• Monitor page used for VMI related events
• Two additional pages: memory sharing and
paging

15. vm_event & mem_access & monitor
● Let's keep track of subsystem names
● vm_event is the underlying request/response
mechanism
● mem_access memops control EPT
● monitor_op domctls control all other optional
VM execution traps

16. Event delivery structures in 4.6
● Defined in xen/vm_event.h public header
● Easily extendable and versioned
● No more hackery
● Event response can trigger specific behavior
without additional hypercalls
• Trigger emulation, singlestepping, swap altp2m...

17. VMI with Xen on ARM
● ARM has two-stage paging similar to EPT
● mem_access implemented for 4.6
● Some caveats:
• No singlestepping?
• Can be worked around but it's a pain
• Split-TLB ambiguities

18. ARM mem_access
● ARM PTEs have fewer software
programmable bits as compared to EPT
● ARM mem_access requires maintaining a
Radix-tree to keep track of PTEs with
custom permissions
● Radix-tree keyed with GPA

19. ARM mem_access
● For a 2nd
stage violation ARM provides the
faulting GVA
● GPA only provided if fault happened during
1st
stage pagetable walk
● Xen needs to translate GVA to GPA to
perform Radix-tree lookup

20. ARM mem_access
● Native CPU instructions to perform GVA to
GPA translation
● Performs lookup as data-fetch access
● What if we trapped an instruction-fetch
access?
• In-guest translation hits iTLB
• Xen hits dTLB

21. ● Split-TLB is a real rootkit problem
• ShadowWalker, MoRE, etc..
● Guest can load the iTLB with rootkit page
and dTLB with benign page
● Flushing the TLB does not help, iTLB
translation may be lost if PT no longer
represents the cached translation
ARM Split-TLB problem

22. ● Execution tracing with mem_access may be
problematic
● Use Secure Monitor Call (SMC) instruction
injection!
● Similar to 0xCC injection on x86
● TODO
ARM future work

23. ● altp2m is primarily designed to be used with
Intel #VE
● VMCALL instruction to perform EPTP
switching from the guest
● Hybrid VMI
● KVM events
x86 future work

24. ● Why aren't we using git pulls?
• Patches in mailinglist without branch-off point
specified
• Carving patches from mbox is a pain
• Start providing a public git branch for your
series!!
Lessons learnt

25. ● Provide build-testing for the community
• It's a waste of time to wait for review on
something that's broken
• Check for style issues automatically?
• Travis-CI is OK but can time-out on large series
• https://github.com/tklengyel/xen/tree/travis
Lessons learnt

26. Thanks!
Tamas K Lengyel
tamas@tklengyel.com
tlengyel@novetta.com
@tklengyel
LibVMI http://libvmi.com
DRAKVUF http://drakvuf.com